Base sheet for strippable wall covering and process for making

ABSTRACT

A base sheet made on a paper machine for use in the manufacture of strippable wall coverings, the process of making it, and strippable wall coverings including it. The base sheet is characterized by having throughout a non-continuous membrane formed from a self-reactive vinyl acrylic terpolymer latex comprising butyl acrylate, vinyl acetate, and a self-reactive comonomer and having certain specified properties and characteristics.

UNITED STATES PATENTS 3/1961 Maeder et a1 117/33 D United States Paten11 1 1111 3,840,428 Ring et a1. 1 51 Oct. 8, 1974 [5 1 BASE SHEET FORSTRIPPABLE WALL 2,976,167 3/1961 Maeder et a1 117/161 COVERING ANDPROCESS FOR MAKING 3,083,172 3/1963 260/29.6 3,144,380 8/1964 Drennan162/164 [75] Inventors: Michael Ring, Warw1ck; Joseph F. 3,149,0239/1964 Bodendorf e181. 162/135 Sheehey, South Glens Falls, both of3,184,373 5/1965 Ar1edter..... 162/1'68 N.Y. 3,190,789 6/1965 Taylor162/181 3,235,443 2/1966 Greenman et a1. 162/135 Asslgneei llltematlonalPaper Company New 3,275,469 9/1966 Streit 117/76 York, 3,275,469 9/1966Streit 117/161 [22] Filed: June 2, 1972 [21] Appl. No.: 259,343 PrimaryExaminer-George F. Lesmes Assistant Examiner-William R. Dixon, Jr.Rained Apphcatlon Data Attorney, Agent, or Firm-Walt Thomas Zielinski[63] Continuation of Ser. No. 32,365, Apnl 27, I970, abandoned, which isa continuation-in-part of Ser. No. 803,523, Jan. 29, 1969, abandoned,which is a continuation-in-part of Ser. No. 531,317, Dec. 1, 1965,abandoned. ABSTRACT [52] US. Cl 162/146, 161/146, 161/232, A base sheetmade on a paper machine for use in the 161/233, 161/251, 162/168,162/184, 260/851 manufacture of strippable wall coverings, the process[51] Int. Cl D21h 5/12 of making it, and strippable wall coveringsincluding [58] Field of Search 162/ 135, 168, 181; it. The base sheet ischaracterized by having through- 161/167, 232, 233, 251; 260/80, 81,29.6; out a non-continuous membrane formed from a self- 117/161 A, 161L, 161 LN, 161 VT, 155 R, reactive vinyl acrylic terpolymer latexcomprising 155 L, 155 A butyl acrylate, vinyl acetate, and aself-reactive comonomer and having certain specified properties and [56]References Cited characteristics.

2 Claims, N0 Drawings This application is a continuation of U.S. Ser.No. 32,365, filed Apr. 27, 1970 and now abandoned, which was acontinuation-in-part of U.S. Ser. No. 803,523, filed Jan. 29, 1969 andnow abandoned. U.S. Ser.No. 803,523 was a continuation-in-part of U.S.Ser. No. 531,317, filed Dec. 1, 1965 and now abandoned.

This invention relates to strippable wall covering, that is, a wallcovering-which, after having been hung on a wall for an extended period,may be readily pulled from the wall in large sheets, leaving the wallintact and clean, in condition for a subsequent application of anotherwall covering. More specifically, this invention relates to the basesheet used in the manufacture of strippable wall covering.

Customarily, a wall covering manufacturer applies a coating, orcoatings, to a base sheet and prints a pattern thereon. This coated andprinted sheet can be embossed. As is made clear in U.S. Pat. No.3,212,957, issued Oct. 19, 1.965, the base sheet can also be combinedwith other. wall covering materials such as paper, cloth, wood veneer,plastics, etc., but, normally the side of the base sheet which is to beput in contact with the wall receives no further treatment other thanthe paste used in the hanging operation. (For a prepasted grade, thepaste is applied by the wall covering manufacturer.) The base sheet is,then, the component of the complete construction which is principallyresponsible for the strippability of the wall covering.

It is a purpose of this development to provide a base sheet which meetsall the requirements for manufacture and end use of a strippable wallcovering. it is a further purpose to make such a base sheet frominexpensive materials on a conventional paper machine at a' highproduction rate.

It is advantageous for a wall covering to be readily strippable from awall in continuous strips without damage to itself or to the wallsurface; Upon stripping, such wall covering does not rupture, causingfragments of it to remain on the wall surface, nor does it causefragments of the wall to be separated from the wall and be retained uponthe wall covering. And, to obtain the desired strippability, the wallcovering must bestronger than the adhesive bond between it and the wallwhen conventional wall covering adhesives, such as wheat paste, are usedto make the wall covering adhere to the wall. Or, in other words, thewall covering must possess a high level of wet tensile strength and ofwet edge tear strengh. See, for example, U.S. Pat. No. 3,212,957, issuedOct. 19, 1965. In addition, the side of the wall covering which is to beapplied to the wall must have a high level of water resistance toprevent the penetration into it of water-based adhesives.

A strippable wall covering must also satisfy the requirements expectedof conventional wall coverings. It must not slide nor sag on the wall.It must remain firmly and uniformly attached to the wall for extendedperiods. It must not curl when it is hung. It must possess the properdrape" and hang for ease of application.

Again, the base sheet of a strippable wall covering must be suitable forsubsequent converting operations. In a typical wall coveringmanufacturing operation, the base sheet is coated with a ground coat";this coating normally contains a pigment such as clay and a waterbasedadhesive and is applied as an aqueous system and subsequently dried. Theground coat is normally printed and subsequently covered with anadditional transparent coat of polyvinyl chloride or the like. Thus, thebase sheet must have a smooth finish and be satisfactorily devoid offuzz. It must readily accept a water-base coating; it must lend itselfto a good bond with the ground coat. The base sheet must be sufficientlyflexible, so that the dried ground coat does not crack when the coatedsheet is folded. The base sheet must not contain materials which wouldcause discoloration or other undesirable change in the applied coatingsupon aging. The degree of stretch of the base sheet must be limited lestit cause the problem of misregister in the printing operation on theground coat. The base sheet must not curl excessively during a coatingoperation. It must accept deep embossing.

Then too, the wall covering must have high opacity to preventshow-through of a pattern of a material upon which may be applied andmust retain a high proportion of this opacity when it is wet, to satisfythis requirement when the sheet is wetted at the time of application.The base sheet should have a high brightness so that it does notadversely affect the brightness of the finished wall covering andconsequently its suitability for printing a pattern on a brightbackground. The wall covering is normally expected to bestain-resistant; the base sheet should not contain materials which willdetract from this property.

Strippable wall coverings have previously been provided. These wallcoverings, however, have disadvantages.

Strippable wall coverings have been prepared from vinyl films, coatedwith rubber-based pressure-sensitive adhesives. Vinyl film of sufficientweight in itself to be the covering is expensive; rubber-basedpressuresensitive adhesives are many times more expensive thanconventional wallpapaer adhesives. There is a further disadvantage ofpressure-sensitive adhesives; A paperhanger must be able to move, orslide, paper laterally on a wall to align one strip with the strip whichhe has applied immediately before. With a pressuresensitive adhesive itis very difficult to achieve this lateral movement. Another disadvantageof pressuresensitive adhesives is their failure with age; the rubbercomponent degrades through oxidation-Upon stripping a wall coveringapplied with such an aged adhesive, a considerable residue of theadhesive remains on the wall. This residue is very difficult to remove;normally an organic solvent is required for effective removal.

Strippable wall coverings have been made by laminating conventionalhanging stock to woven textile scrim, or by forming a sheet of whichscrim is an inte- 'gral part; the scrim provides the strength necessaryfor strippability. Such a material has certain disadvantages. Thelamination process is normally slow; production of a sheet of whichscrim is an integral part also is normally slow. The scrim is expensive.Unbleached scrim, which is less expensive than bleached scrim, containsobjectionable dirt. The only embossing pattern readily usable is thatwhich is imposed by the geometry of the scrim itself.' The surfaceirregularities caused by the scrim make uniform printing very difficult.In addition, scrim-containing papers are stiff. A paperhanger normallyfolds long strips of wall covering to facilitate handling and, when ascrim-containing sheet is folded,

' the coating frequently cracks, causing unsightly marks.

Strippable wall coverings have been made by applying a coating ofpolyvinyl chloride to a closely woven fabric. The principal disadvantageof such a product is the extremely high cost of the fabric. Becauseof'the texture of the fabric, printing is frequently a problem with suchmaterials. Pasting of such wall coverings-is frequently a problem; thepaste tends to fill the voids of the fabric instead of forminga filmover the entire surface.

Strippable wall coverings have been made with base sheets containinglong fibers of synthetic polymers in mixture with a major portion ofconventional papermaking fibers, the whole fiber furnish being bondedtogether with a resin. The synthetic fibers, which may be up to an inchin length, provide the impression of a'fabric. These synthetic fiberscause many papermaking difficulties which make the production rate ofsuch a sheet very slow. In addition, such fibers are very expensive.

The strippable wall covering basesheet of this invention is produced onconventional papermaking equipment and, broadly, the sheet is formedfrom a suspension of a mixture of conventional papermaking or cellulosicfibers, synthetic fibers (optional), and a filler or pigment; ispartially dried; is saturated with a synthetic latex; and, is then driedto a moisture content normal for paper products. This sheet has beenproduced on a Roto-former machine, but it can be made on a Fourdrinierpaper machine. The formed and partially dried sheet can be saturatedwith the synthetic latex on a conventional size press which is anintegral part of the paper machine, but it can as well be saturated onother conventional saturating equipment which is either integral withthe paper machine or separate from the paper machine.

In the product and process of the present invention, a variety ofmaterials can be used. Thus, for example, strong, bleached cellulosicpulps of the type commercially available under the trademark Celgar orof the type reclaimed from waste polyethylene-coated (polybroke) paperused in the manufacture of milk cartons or the like can be resorted tofor the papermaking fibers. The synthetic fibers, if used at all, can berayon, nylon, Orlon, or Dacron and they can be used in amountsrepresenting as much as 50 percent by weight of the fiber furnish in thebase sheet of the present invention. As a filler or pigment, a titaniumdioxide material such as the commercially available Titanox RA-SO can beused, as can any one of a number of others familiar to those skilled inthe art.

It is distinctly useful to employ a wet strength agent such as one ofthe commercially available melamineformaldehyde resins, e.g., ParezResin 607. And, if some of the Parez Resin 607 is used together wihtanother melamine resin such as Parez Resin 613, a filler retention agenthaving additional advantageous properties is at work. The combination ofa melamineformaldehyde resin (e.g., Parez Resin 607) and a highlyreactive melamine resin of the methylated methylol melamine type (e.g.,Parez Resin 613), have proven especially worthwhile and beneficial,provided both resins are added to the stock prior to the formationtherefrom of a sheet.

So far as can be ascertained, the presence of the Parez Resin 607 whenthe sheet is formed from stock containing it insures a sufficient degreeof wet strength in the sheet to permit complete submersion thereof,without disintegration, in a bath of the completed latex. And, uponsubsequent drying of the sheet, the presence of ,Parez Resin 613 whenthe sheet is formed from stock containing it insures a reaction with thelatex which produces a tough, non-continuous, water resistant membranethroughout the sheet. Thus, by adding the methylated methylol melamineto the stock at the wet end and the latex at the size press in theprocess of the present invention, there is achieved an in situ reactionbetween the melamine and latex which forms a three dimensional membranethroughout the sheet.'

the sheet and controls its strippability by permitting some, yetcontrolling the degree of, penetration into the sheet of thepaperhangers paste.

By contrast, if the Parez Resin 613 were added to the latex emulsion atthe size press, as is fairly conventional, rather than to the stockslurry from which the sheet is later formed, a water resistant skin orfilm would be formed on the surface of the sheet. This would prevent theproper adhesion of the wall paper paste to the sheet and the subsequentadhesion of the sheet to a wall. Clearly, then, the point of addition ofthe Pal-e2" Resin 613 is significant.

As for the correct synthetic latex to use in the process and product ofthe present invention, the work thus far undertakenhas centered in thearea of the acrylic resins, particularly the self-crosslinking acrylicresins. The latter type of resins is discussedin the American DyestuffReporter for Mar. 5, 1962, as well as other articles in the technicalliterature which are available. These latter materials are produced byproducing copolymers with crosslinking reactive groups constructed intothe polymer backbone, so that the resin becomes self-reactive withoutresort to an-external crosslinking agent.

The common acrylic esters (methyl, butyl, etc.) are essentiallynon-reactive. In order to facilitate crosslinking, it is necessary toincorporate into the polymer a minor proportion of an appropriatefunctional monomer. These reactive monomer groups need not.necessarilybe members of the acrylicfamily, are usually distributed randomly alongthe polymer chain, and permit crosslinking of adjacent chains and theformation of a threedimensional polymer network. They are made to reactat the proper time either with the aid of an external crosslinkingagent, the use of acid catalysis, and v curing or, if the necessaryfunctionality has been built into the basic polymer, with the use ofacid catalysis and curing only. See the American, Dyestuff Reporter forSept. 2, 1963 at page 44.

To date, the best synthetic latex for the purposes of this invention,and the one which is clearly preferred, is a commercially availablematerial which its manufacturer calls a self-reactive vinyl acrylicterpolymer latex and sells as an aqueous dispersion containing about 45percent to 50 percent total solids and anionic or non-ionic surfactants.The use of the surface active agents to disperse the acrylic monomerstherein during emulsion polymerization is said to result in a productofsuperior chemical stability, particularly with respect of 3 to 4minutes at 130C. is usually necessary to effect a complete cure. Oxalicor citric acid or ammonium salts of these are suitable catalysts foraccelerating the curing.

More particularly, the preferred terpolymer em- 5 is in the range ofabout to 35 parts by weight, and 15 the proportion of self-reactivecomonomer is in the range of about 1 to 10 parts by weight. Further,such terpolymer is characterized by a second order transitiontemperature (i.e., a temperature below which the polymer changes from arubbery to a plastic material) of themagnitude of about -36C.; a tensilestrength of the magnitude of about 240 psi and, after curing seconds at130C., about 275 psi; and, an elongation of the magnitude of about 925percent and, after curing 30 seconds at 130C., about 300 percent. Here,tensile and agitated at ambient temperature to achieve approximately 20percent solids. Ammonium chloride, to serve as a curing aid, is added inaqueous solution. Then, the entire saturant is pumped to the size presspond and applied to the wall covering sheetin the conventional manner.

Attached Table 1 summarizes the data drawn from the preparation of threequalities of wall covering materials made in accordance with the presentinvention.

0 Such table, then, describes three examples of such coverings in' termsof both their constituents and their properties. It does not, of course,exhaust the possible combinations of ingredients which properly comewithin the scope of the present invention. Neither does it, forinstance, limit the quantity of synthetic latex and curing aid which canbe used. Indeed, it is to be clearly understood that the lower limit ofsuch quantity is determined primarily by the strength requirements ofthe wall covering sheet being made and the upper limit is determinedprimarily by the application equipment available and the furtherhandling of the sheet. If, for instance, it is desired to ground coatthe sheet (cf., U.S. Pat. No. 3,212,957, column 2), care must be takento avoid the use of so much of the synthetic latex as will 25 inhibitproper adhesion between the ground coat and Alum added tp pHapproximately 4.5.

'Tensile Tester using 3 mil (approx.) films dried at 72F.

and elongatlon values were obta ned wlth-an lnstron sheet.

TABLE I High Medium Low White White White Grade Grade Grade BEATERFURNISH (Parts) Celgar" bleached kraft 46.5 Polybroke Pulp 46.5 Rayon l57 5 Parez Resin 613 1.5 1.3 1.3 Parez Resin 607 0.5 0.5 0,5 TitanoxRA-50 12 5 3.5 Alum SATURANT o1 finished sheet) Terpolymer (Resyn 2873)20 20 20 Ammonium Chloride 0.3 0.5 05

TEST RESULTS Dry Dry Wet Dry Wet Basis Wt. (24X36-500). lb 82 84.3 85.0

Cali er, mils 8.0 8.2 8.7 Mul en, psi 69.8 57.6 Tensile, lb/in. MD 44.250.0 19.0 37.6 19.4 CD 21.7 13.9 6.3 13.9 6.0 Stretch. MD 3.5 9.5 3.412.2 CD 9.1 14.6 8.5 18.5 Tear. 3 With Gr 153 134 142 Across Gr 159 208202 Edge tear. g MD 5300 4700 6000 CD 6950 5600 5750 Fold MD 6100 2735CD 525 Porosity, sec/100 cc 40 15 Brightness, F 81.5 72.5 67.9 W 82.172.1 67.8 Opacity, 91.4 94.1 91.8 lnk Penetration F 1000+ 1380 3600 w3600+ 2700 What is claimed is:

l. A process for manufacturing a base sheet for strippable wall coveringcomprising the steps of (a) forming a paper sheet from cellulosic fibersand from about 0 percent to 50 percent by weight synthetic fibers filledwith pigment in the presence of a filler retention agent comprising amelamine-formaldehyde resin and a methylated methylol melamine resin;(b) partially drying the formed paper sheet; saturating the formed'paper sheet with not more than about 20 percent by weight of aself-reactive vinyl acrylic terpolymer latex comprising butyl acrylate,vinyl acetate, and a third monomer containing polyfunctional groupscapable of crosslinking with similar groups in adjacent chains whereinthe proportion of the butyl acrylate is in the range of about 65 to 85parts by weight, the proportion of the vinyl acetate is in the range ofabout 15 to 35 parts by weight, and the proportion of the third monomeris in the range of about 1 to parts by weight and characterized by asecond order transition temperature the process of claim 1.

1. A PROCESS FOR MANUFACTURING A BASE SHEET FOR STRIPPABLE WALL COVERINGCOMPRISING THE STEPS OF (A) FORMING A PAPER SHEET FROM CELLULOSIC FIBERSFILLED FROM ABOUT 0 PERCENT TO 50 PERCENT BY WEIGHT SYNTHETIC FIBERSFILLED WITH PIGMENT IN THE PRESENCE OF A FILLER RETENTION AGENTCOMPRISING A MELAMINEFORMALDEHYDE RESIN AND A METHYLATED METHYLOLMELAMINE RESIN; (B) PARTIALLY DRYING THE FORMED PAPER SHEET; (C)SATURATING THE FORMED PAPER SHEET WITH NOT MORE THAN ABOUT 20 PERCENT BYWEIGHT OF SELF-REACTIVE VINYL ACRYLIC TEROLYMER LATEX COMPRISING BUTYLACRYLATE, VINYL ACETATE, AND A THIRD MONOMER CONTAINING POLYFUNCTIONALGROUPS CAPABLE OF CROSSLINKING WITH SIMILAR GROUPS IN ADJACENT CHAINSWHEREIN THE PROPORTION OF THE BUTYL ACRYLATE IS IN THE RANGE OF ABOUT 65TO 85 PARTS BY WEIGHT, THE PORPORTION OF THE VINYL ACETATE IS IN THERANGE OF ABOUT 15 TO 35 PARTS BY WEIGHT, AND THE PORPORTION OF THE THIRDMONOMER IS IN THE RANGE OF ABOUT 1 TO 10 PARTS BY WEIGHT ANDCHARACTERIZED BY A SECOND ORDER TRANSITION TEMPERATURE OF THE MAGNITUDEOF ABOUT -36*C., A TENSILE STRENGTH OF THE MAGNITUDE OF ABOUT 240 PSI, ATENSILE STRENGTH AFTER CURING 30 SECONDS AT 130*C. OF THE MAGNITUDE OFABOUT 275 PSI, AN ELONGATION OF THE MAGNITUDE OF ABOUT 925 PERCENT, ANDAN ELONGATION AFTER CURING 30 SECONDSS AT 130*C. OF THE MAGNITUDE OFABOUT 300 PERCENT; AND, (D) DRYING THE SATURATED SHEET WHICH H REACT THEMETHYLATED METHYLOL MELAMINE RESIN AND THE LATEX TO PRODUCE THE BASESHEET WHICH HAS THROUGHOUT A NONCONTINUOUS MEMBRANE FORMED FROM THELATEX.
 2. A base sheet for strippable wall covering made by the processof claim 1.